The present invention relates to a valve for dosing liquids or gases, particularly an injection valve for fuel injection system in internal combustion engines.
More particularly, the present invention relates to such a valve which has a housing with a dosing opening, a valve needle which controls the dosing opening, a valve closing spring which retracts the valve needle to its closing position in which it blocks the dosing opening, a piezoelectric adjusting member formed as a piezo stack which is longitudinally changeable under the action of a control voltage and is connected at its one end with the valve needle, and at its other end with an abutment displaceable in an axial direction of the piezo stack and formed so that it is spatially fixed relative to the housing during the longitudinal change of the piezo stack under the action of the control voltage.
An injection valve of this type is known in the art and disclosed, for example, in the British patent application No. 2,056,559.
Here, the abutment is formed as a damping piston which is arranged in a fuel-filled damping cylinder. The damping cylinder communicates via an opening with a fuel supply. The damping piston abuts against the piezo stack under the action of the valve closing spring which is arranged in the damping cylinder, and the piezo stack presses the valve needle against the valve seat which surrounds the injection opening and in turn blocks the injection opening. When a control voltage is applied to the piezoelectric adjusting member, the piezo stack contracts and shortens its length by approximately 30 mm. The mass of the damping piston and the damping action of the fuel in the damping cylinder acts so that the abutment for the movement of the piezo stack contraction is always stationarily fixed, and the contraction of the piezo stack does not produce any displacement of the damping piston through the valve closing spring. As a result of this, the entire longitudinal change of the piezo stack is converted into a stroke movement of the valve needle, so that the relatively small adjusting path of the piezo stack is completely used for opening of the injection valve.
When the injection valve is closed, the longitudinal displacements of the piezo stack by temperature changes or length differences of the entire system of valve needle, piezo stack and damping piston, by wear or manufacturing tolerances, produce a respective displacement of the damping piston. Since the housing-fixed abutment of the piezo stack takes place during the valve actuation, these influences are completely compensated and does not affect the stroke path of the valve needle and thereby the injection cross-section released by the valve needle. The dosing and thereby the injection quantity is dependent on temperature, manufacturing tolerances or wear and is approximately constant. A continuous reproduction of the injected fuel quantities is therefore guaranteed.
It should be mentioned that this known construction principle of fixation of the abutment of the piezo stack during the dosing phase cannot be transferred to the piezoelectric control member when the adjusting path is generated by longitudinal expansion of the piezo stack. In these cases the pressure force which acts during the expansion of the piezo stack upon the damping piston causes a displacement which is considerable as compared with the relatively small adjusting path of the piezo stack of typically 20 mm. A partial quantity of the fuel located in the damping cylinder can be approximately unthrottled and thereby displaces very fast through the opening in the fuel supply. Only during strong throttling of the fuel discharge it cannot be assumed that small quantities cannot be discharged sufficiently enough or the liquid volume accommodated in the damping cylinder is completely without gas influence and thereby incompressible. A high mass inertia of the damping piston can be used only conditionally and considerably increases the structural volume of the valve.
Any, even insignificant displacement of the damping piston and thereby of the housing-fixed abutment of the piezo stack during the valve opening leads to a changing stroke path of the valve needle and to a change of the opening--or dosing cross-section of the valve. The dosing of especially small quantities is thereby inaccurate and changes in each dosing phase. Thereby it is impossible to achieve either the required dosing accuracy or the required dosing constancy in dependence of the valve opening time.